Abstract

We experimentally and theoretically studied the phenomenon of thermal
emission from nonvolatile liquid surface coatings following heating with a
pulsed CO2 laser. The effects of thermal diffusion
across the liquid–air and liquid–substrate interfaces as well as
the full absorption spectrum of the liquid are addressed theoretically. The
differential temporal and intensity characteristics of the thermal emission
signal from the heated surface coating, resulting from the differential heat
deposition profile for on- and off-resonance excitation, are shown to be
useful for the purposes of identifying different surface contaminants. The
application of this technique to standoff thermal imaging of contaminated
surfaces is discussed.

Figures (11)

Absorption coefficients of MS over the spectral
region 2.2–20 µm. These data were put together from individual
transmission spectra taken over path lengths of 6, 25, 50, and 80 µm. The
spectrometer was a Fourier transform device with a resolution of 4
cm-1.

Example of the solution to the diffusion equation for
infinitely thick liquid and air layers with an initial exponential heat
distribution in the liquid. The excitation absorption coefficient was 0.5 µm-1 and curves (1)–(3) correspond to
times of 1, 10, and 100 µs after the laser heating pulse. The liquid
assumed here was water, and the associated physical constants are given in
Table 1.

Examples of the diffusion equation solution as a
function of time for the case of a liquid layer sandwiched between air and a
substrate: (A) the effect of a glass substrate and (B) the effect of a steel
substrate.

Theoretical temporal evolution of the spectral
radiance of MS following off-resonance pulsed excitation at 9.52 µm. The
curves correspond to the same times as given in
Fig. 6. All curves appear approximately overlaid for this
scaling.

Integrated radiance across the 8–14-µm
spectral band as a function of time for various cases: ■, decay following
on-resonance pulse heating; ●, decay following off-resonance pulsed
heating; (1) blackbody at a constant 305 K; (2) blackbody at a constant 302 K;
(3) blackbody at a constant 300 K; (4) MS at a constant 300 K.

Integrated radiance across the 3–5-µm
spectral band as a function of time for various cases: ■, decay following
on-resonance pulse heating; ●, decay following off-resonance pulsed
heating; (1) blackbody at a constant 305 K; (2) blackbody at a constant 302 K;
(3) blackbody at a constant 300 K; (4) MS at a constant 300 K.

Experimental and theoretical data for the decay of
the PPTR signal after pulsed heating at different wavelengths: ■,
theoretical data for λ = 9.2 µm; ▲, theoretical data for λ
= 9.52 µm. Curves represent experimental data for the three
wavelengths shown. The experimental data were sampled at 500
kHz and the traces shown are the result of five averages. The
signal was filtered through a low-pass filter with a cutoff frequency of
1.0 MHz.